A long time has passed since a power element in the general industrial field, was adopted in general household electrical appliances, equipment for public use, and the like. This kind of power element has recently been required to operate with high efficiency, at high speed and with a low loss, and tends to perform its switching operation at higher and higher speed.
One example of a prior art self-arc-extinguishing type power element driving unit will be described with reference to FIG. 1. The unit illustrated in FIG. 1 is one for driving a typical IGBT 1 as a self-arc-extinguishing type power element. A signal supply circuit 7 generates an on/off signal Id for driving the power element as shown in FIG. 3A. The output of the signal supply circuit 7 is supplied to the gate of the IGBT 1 through a photocoupler 5, an amplifier 6, and a power amplifying circuit including a driving gate resistor 2 and NPN type transistors 3 and 4. The reason for employing the photocoupler 5 is that the IGBT 1 has to be insulated from the driving unit side since the IGBT 1 is connected to a main circuit and turned on/off at high voltage and changes its emitter potential to a high potential in an example of an inverter bridge. Since, furthermore, the insulation output of the photocoupler 5 is a small signal one, it needs to be amplified and thus the amplifier 6 is provided to do so.
An operation of the driving unit shown in FIG. 1 will be described with reference to FIGS. 3A to 3F. The power element driving on/off signal Id, as shown in FIG. 3A, is generated from the signal supply circuit 7 to turn on/off the primary side of the photocoupler 5. As a result, when the photocoupler 5 is turned on, a current flows into a light-emitting diode which is a light-emitting section of the photocoupler 5, and thus the light-emitting section emits light. If a light-receiving section of the photocoupler 5 receives the signal, an insulation output 5a flows. The potential of output 5a of photocoupler 5 is equal to logic "0" when the photocoupler 5 is turned on. The output 5a of the photocoupler 5 is supplied to the amplifier 6 and power-amplified by the transistors 3 and 4. Then, as shown in FIG. 3C, the outputs of the transistors 3 and 4 or the potential of the gate resistor 2 is set to a value of approximately +V.sub.CC or -V.sub.CC relative to the emitter potential of the IGBT 1. The potentials +V.sub.CC and -V.sub.CC of the gate resistor 2 are set by power supplies 8 and 9.
An equivalent circuit of the IGBT 1 is illustrated in FIG. 2 as a typical example of the power element. The gate input section of the IGBT 1 constitutes an FET (Field Effect Transistor) 1a, and the gate-to-source thereof is high in impedance but has capacitive characteristics.
Consequently, the gate potential of the IGBT 1 has a capacitive potential and thus varies with the gate resistor 2 and the time constant of input capacitance. The IGBT 1 is turned on/off in response to the on/off signal generated from the driving unit and thus the respective elements of the IGBT 1 make/brake the current of the main circuit based on the waveforms indicated in FIGS. 3D to 3F. FIG. 3D shows a gate-to-emitter potential V.sub.GE, FIG. 3E represents a collector-to-emitter potential V.sub.CE and FIG. 3F illustrates a collector current.
However, in the prior art self-arc-extinguishing type power element driving unit, a current variation di/dt at the time of ON/OFF of the current flowing into the main circuit is raised as one problem. There is an inverter as a device to which the power element is applied. If such an inverter requires a large amount of electric power, it always has an inductance in the main circuit in view of the constitution of hardware.
For this reason, a surge voltage is generated based on Ldi/dt by the current variation di/dt at the time of switching, and an EMI (Electron Magnetic Interference) is caused due to the surge voltage.
As described above, a voltage variation dV.sub.CE /dt of the collector-to-emitter potential V.sub.CE at the time of switching causes a problem of EMI.
Especially in the case of a power element performing a high-speed switching operation, dV/dt is increased to cause a problem of a higher-frequency electromagnetic interference.